The present invention relates to a glass-type transparent substrate coated with a stack of thin layers and to a process for the production of a coated substrate. In particular, the coated substrate exhibits properties of low emissivity and low haze.
Tin oxide-based layers are known. For example, layers based on tin oxide doped with fluorine are known for their properties of low emissivity and electrical conductivity. On one hand, these materials provide an increased reflection of the electromagnetic radiation at wavelengths in the range between 3 and 50 μm and therefore allow reflection of infrared radiation. On the other hand, antimony doped tin oxide layers are known both for their property of low emissivity and for their more pronounced absorption in wavelengths in the visible range than fluorine doped tin oxide layers and are used for antisolar and/or thermal insulation applications.
It is also known that tin oxide-based layers deposited onto glass by pyrolysis (in vapour phase—chemical vapour deposition (CVD), in liquid phase (sputtering) or in solid phase (powder sputtering)) generally give a whitish “haze”(Applied Surface Science, 185 (2002) 161-171, J. Szanyi “The origin of haze in CVD tin oxide thin films”). This haze is caused by scattering of the light. This article describes, for example, SnO2:Sb layers with a thickness of 264 and 215 nm, which respectively give a haze of 1.55 and 3.95%.
The standard ASTM D 1003-61 defines the “haze” as the percentage of transmitted light, which, while passing through the sample, deviates from the incident beam by an angle of more than 2.5°.
In order to improve the infrared reflection properties of the tin oxide-based layer, it is generally necessary to increase its thickness. However, the greater the thickness of the deposited tin oxide layer, the more the haze increases. Typically, a tin oxide layer of 500 nanometres gives a haze of 2 to 20%. This haze gives a whitish aspect to the vision in transmission and is therefore redhibitory.
Moreover, it is known that the industrial production of such products causes undesirable variations or non-homogeneity. For example, some zones or isolated points can present an increased haze. These localised faults may be visible to the naked eye and render the product unacceptable.